In downhole wellbores the flow paths through the main bores of the well and other flow conduits are typically controlled by valves. The valves typically close and open against a seat which provides a sealing face to engage with a movable member in the valve and prevent fluid flow. Typically the seat is faced with a resilient material that conforms to the sealing face of the valve to create the seal.
Problems arise when the seat of the valve is damaged by the movement of the moveable member between its open and closed configurations. Typically, in a ball valve, the valve moves between its open and closed configurations by rotation of the ball, to rotate a bore in the ball in and out of alignment with the bore of the conduit in which the ball valve is located. As the bore of the ball rotates out of alignment with the bore of the tubular, the edge of the bore can sometimes tear or otherwise damage the seat against which the ball seals, possibly tearing the resilient material on the face of that seat.
This is particularly problematic when the ball valve needs to cut a wire or other elongate member that is passing through the bore as it moves between the open and closed configurations, because the inner edge of the bore through the ball typically becomes damaged by shearing of the wire, and the damaged edge then rakes across the more delicate seating surface on the seal, typically causing tears and other damage to the softer material used on that component. When the ball valve has closed and the bore of the valve is out of alignment with the bore of the tubular, the sealing surface of the ball frequently does not seal adequately against the damaged surface of the seat, and leaks then arise when the stroke has been completed.